Our goal is to study the molecular events related to heavy chain class switching in a murine B cell lymphoma, BCL1. In the first grant the primary focus was to obtain inducible versions of BCL1 that displayed various developmental phenotypes and to study the DNA rearrangements accompanying ligand-induced differentiation. Two major results from these studies have provided the basis for the present proposal: First, BCL1 can undergo differentiation from IgM+IgD expression to stable IgM+IgG1, allelically excluded double-production without DNA rearrangement of the heavy chain constant region genes. Second, certain T cell-derived supernatants contain an activity (BCDFGamma) that induces specific expression of IgG1 by interaction with a cell surface receptor, putatively identified by a monoclonal antibody. A primary objective, made possible by the unique BCL2 line, is to determine the detailed molecular mechanism for dual Mu and Gamma1 isotype production. Using BCL2 clones which express different ratios of Mu to Gamma1 we will determine the point at which regulation of the relative isotype levels and their membrane and secreted forms is exerted. Dual IgM and IgG production may be an intermediate phenotype (e.g., memory cell) to terminal differentiation. Therefore we will attempt to induce BCL2 nonsecreting subclones to undergo further differentiation using BCDFGamma, its anti-receptor antibody and other ligands. To test the mechanism of BCDFGamma action, we will determine if receptor-positive BCL1 cells transfected with productively rearranged Gamma1 and Gamma3 vectors will undergo ligand induced differentiation. Finally, a study of the chromatin alterations in BCL1 is designed to test whether there is a molecular basis for the "precommitted" switch to Gamma1 observed in these cells.